1. Field of the Invention
The present invention relates to an apparatus and a method for cleaning a screen of a screen printing machine.
2. Related Art Statement
Screen printing is carried out in such a manner that a substrate such as a printed circuit board ("PCB") is held in contact with one of opposite surfaces of a screen and a printing material ("PM") such as creamed solder is squeezed, from the other surface of the screen, into a number of through-holes formed through the thickness of the screen. Accordingly, as the number of printings increases, the contact surface of the screen with which the substrate is contacted, the squeeze surface from which the PM is squeezed, and the inner surfaces of the through-holes are stained little by little with the PM adhered thereto. If this screen is used as it is for effecting additional printings, there may arise some problems, e.g., that only an insufficient amount of PM is squeezed into the through-holes and accordingly an insufficient amount of PM is provided or printed on the substrate, or that the substrate is strained with the PM adhered to the screen. Therefore, the screen is cleaned, e.g., after the screen is used for printing the PM on a predetermined number of substrates, if it is speculated from a detected amount of the PM printed on a substrate that the amount of PM printed on the substrate is insufficient because the screen is strained with the PM adhered to, e.g., the through-holes, or when a screen printing operation is ended.
One of known screen cleaning devices cleans a screen by applying ultrasonic vibration thereto. This screen cleaning device includes an ultrasonic vibrator and a washing-agent ("WA") supplying device which supplies a WA to a space between the screen and the vibrator. The vibrator is contacted with the screen, the WA is supplied to a small space left between the screen and the vibrator, and the screen is vibrated by the vibrator. Consequently the PM adhered to the contact and squeeze surfaces and through-holes of the screen is removed and is diffused into the WA. Thus, the screen is washed.
However, the above-indicated prior screen cleaning device suffers from the problem that since the ultrasonic vibrator is directly contacted with the screen, the screen may be damaged. In addition, since the PM is adhered to the vibrator, the capability of washing of the vibrator decreases as the number of washings increases. Although the washing capability may be restored by cleaning the vibrator, it is very cumbersome to clean the vibrator.